木本植物木质部栓塞修复机制的探讨

Discussion on the embolism repairing in xylem of woody plants

该文用几种木本植物茎做切片实验,探讨木本植物木质部栓塞的修复机制.研究发现:当脱水样品被复水后,在表面张力的作用下,其管状细胞中的空气立即回缩,形成长形或球形气泡;此后,在比较小的管状细胞中的气泡逐渐缩小直到消失;大的管状细胞中的气泡最初会伸长,然后缩短,直到消失;胞外球形泡也会一个接一个地出现;除了来自细胞的切破处,这些胞外球形泡还来自木射线的横断面,其中小些的会缩小,大个的会长大.这些现象说明,样品中的气体并没有如以往所云,溶解到水中之后进入大气.作者认为,空气也许从短半径气泡移动到了长半径气泡中.有两种可能的机制可以解释这一现象,一是根据表面张力附加压强公式,气体会由半径小、压强大的小气泡移动到大气泡;二是根据物理学的统计理论,从小气泡溶解到水分中的空气能够扩散,进入临近的、压强较小的大气泡.两种机制导致样品中小管状细胞栓塞的修复.因此,作者认为,对活体植物,空穴化事件之后,空气难以从木质部移动到大气中,而是从小的管状细胞中运动到了大的管状细胞中或其他低气压区域,从而导致木质部栓塞的修复.

Wood sections of species of plants were made and observed under microscope.When a dehydrated section was rewetted,the air inside its conduits contracted under the force of surface tension for several seconds to form elongated or round bubbles.The elongated bubbles in the smaller conduits shortened till vanished.The authors also discovered that the bubbles outside conduits appeared gradualy one after another and they originated mainly from the cross sections of the wood rays.The smaller ones collapsed and the larger ones grew up gradually.The bubbles in the larger conduits extended at first,and then collapsed and disappeared. It was suspected that air might transfer from the bubbles with short radii to those with large ones,both inside and outside conduits.There are two possible mechanisms to explain the phenomena.First,based on the capillay equation,air can move from the smaller bubble to the larger one.Another reason is that the dissolving air from the smaller bubbles can enter into the adjacent bubbles with larger curvature radii.Therefore,it is suggested that after cavitation events,instead of air moving from xylem into ambient atmosphere,two mechanisms could induce air to transfer from the smaller conduits into the regions with lower pressures.Thus,the embolized conduits in the smaller conduits are repaired.